1. Field of the Invention
The present invention relates generally to an automatic transmission and more specifically to a control circuit for an automatic transmission which features a single valve which provided both timing and pressure level control during transmission downshifting.
2. Description of the Prior Art
JP-A No. 61-175357 discloses a control arrangement which includes a 4-3 sequence valve which is used to suppress select shock when the select lever is moved in a manner which induces a 4-3 downshift. That is to say, the friction elements which are used to produce 4th speed are connected with an accumulator and when a 4-3 downshift is induced, the pressure which holds the friction elements in their engaged state is reduced gently. With this arrangement, until the pressure falls below a predetermined value the 4-3 sequence valve does not change position. As this valve is arranged to control the supply of hydraulic fluid to the overrunning clutch until the fourth speed inducing friction elements are sufficiently depressurized, the overunning clutch cannot be engaged. This arrangement thus obviates the generation of shift shock due to partial engagement of the overruning clutch while the friction elements fourth speed is still capable of transmitting torque.
However, with this prior art arrangement since the overrunning clutch is engaged with unmodified line pressure it is not possible to adjust the amount of torque which is transmitted thereby unless a pressure reducing valve is additionally provied. Unfortunately, the provision of this valve induces the drawbacks of increased size and complexity of the control circuit.